Bistable nematic liquid crystal display device and method for controlling such a device

Abstract

The present invention relates to a display device comprising a bistable nematic liquid-crystal matrix screen with anchoring breaking, characterized in that it comprises: components ( 40 ) capable of switching between an off state and an on state, these components being placed respectively between a drive electrode ( 47 ) associated with each pixel and a display state control link ( 45 ); and means capable of applying, to the input of each aforementioned component ( 40 ), via the state control link ( 45 ), input signals comprising at least two phases separated by a controlled time interval, namely a first phase during which the input signal has an amplitude sufficient to permit breaking of the anchoring of the liquid crystal on the associated pixel, then a second phase during which the amplitude of the input signal is controlled in order to select one of the two bistable states of the liquid crystal, the time interval between the two phases being adapted in order to break the anchoring of the liquid crystal on the said associated pixel before the second input signal phase is applied.

The invention claimed is: 
     
       1. Method for electrically controlling a bistable nematic liquid crystal matrix screen composed of pixels provided in a matrix of rows and columns, comprising a liquid crystal layer between two substrates facing each other, two sets of electrodes placed respectively on the substrates, each pixel having two electrodes forming respectively a drive pixel electrode and a back pixel electrode, the drive pixel electrode being connected to the crossing of one row and one column on one substrate, the back pixel electrode being provided on the other substrate, an array of row conducting tracks and of column conducting tracks, and an array of transistors respectively associated with each one of the pixels,
 said liquid crystal matrix screen including a weak zenithal anchoring orientation layer on one of the substrates, 
 each pixel of the bistable nematic liquid crystal matrix screen being addressed via a respective transistor, each transistor comprising a gate, a source and a drain, 
 each transistor being activated by sequentially scanning the row conducting tracks of the screen, the gate of each pixel transistor being connected to a respective row conducting track, the gate controlling the turning-off or turning on of the transistor by applying address signal, the source of each pixel transistor being connected to a respective column conducting track, the source applying control signal, the drain being connected to the drive pixel electrode, 
 said bistable nematic liquid crystal matrix screen comprising two bistable liquid crystal textures, one texture being uniform or slightly twisted wherein the liquid crystal molecules being at least approximately parallel to one another, and the other texture differing from the first texture by a twist of about +180° or −180°, 
 said electrically controlling consisting in switching each pixel between the two bistable liquid crystal textures, 
 
       the method comprising successively in time a first phase, an intermediate phase, and at least one texture control phase,
 the first phase comprising:
 applying to the gate of the transistors of respective pixels corresponding to a common row, via said corresponding row conducting track, an address signal in order to turn on the transistors of the common row, allowing the control signals applied to the source of the same transistors in synchronism with the address signal to appear on the drain of said transistors and consequently on the associated drive pixel electrodes, said control signals having a sufficient amplitude to permit breaking of the anchoring of the liquid crystal on said pixels, and 
 then applying to the gate of the transistors of said common row, an address signal in order to turn off the transistors, 
 
 the intermediate phase having a controlled time interval which is sufficient and adapted to break the anchoring of the liquid crystal of the said pixels and wherein several other rows are addressed during said intermediate phase, 
 the at least one texture control phase comprising:
 applying again to the gate of the transistors of said respective pixels corresponding to said common row, an additional address signal in order to turn on again the transistors allowing the control signals applied to the sources of said transistors in synchronism with the address signal to appear on the drain of said transistors and consequently on the associated drive pixel electrodes, said control signals being applied in order to select the final bistable liquid crystal texture, 
 then applying to the gate of the transistors of said common row, an address signal in order to turn off the transistors. 
 
 
     
     
       2. Method according to  claim 1 , wherein the steps of controlling switching each pixel between the two bistable liquid crystal textures comprise a sequence made up of a first phase and a single texture control phase, the first phase and the texture control phase being separated by an intermediate phase. 
     
     
       3. Method according to  claim 1  wherein a discharge resistor is provided respectively at the terminals of each pixel. 
     
     
       4. Method according to  claim 1 , wherein the steps of controlling switching each pixel between the two bistable liquid crystal textures comprise a sequence made up of a first phase and two texture control phases, the first phase and the texture control phases being separated by two respective intermediate phases. 
     
     
       5. Method according to  claim 4 , wherein said sequence comprises:
 the first phase comprising:
 applying to the gate of the transistors of respective pixels corresponding to a common row, via said corresponding row conducting track, an address signal in order to turn on the transistors of the common row, allowing the control signals applied to the source of the same transistors in synchronism with the address signal to appear on the drain of said transistors and consequently on the associated drive pixel electrodes, said control signals having a sufficient amplitude to permit breaking of the anchoring of the liquid crystal on said pixels, and 
 then applying to the gate of the transistors of said common row, an address signal in order to turn off the transistors, 
 
 a first intermediate phase having a controlled time interval which is sufficient and adapted to break the anchoring of the liquid crystal of the said pixels and wherein several other rows are addressed during said intermediate phase, 
 a first texture control phase comprising:
 applying again to the gate of the transistors of said respective pixels corresponding to said common row, an additional address signal in order to turn on again the transistors allowing the control signals applied to the sources of said transistors in synchronism with the address signal to appear on the drain of said transistors and consequently on the associated drive pixel electrodes, said control signals being applied in order to select the final bistable liquid crystal texture, 
 then applying to the gate of the transistors of said common row, an address signal in order to turn off the transistors, 
 
 a second intermediate phase having a controlled time interval, 
 a second texture control phase comprising:
 applying again to the gate of the transistors of said respective pixels corresponding to said common row, an additional address signal in order to turn on again the transistors allowing the control signals applied to the sources of said transistors in synchronism with the address signal to appear on the drain of said transistors and consequently on the associated drive pixel electrodes, said control signals corresponding to a reset to zero signal, and 
 then applying to the gate of the transistors of said common row, an address signal in order to turn off the transistors. 
 
 
     
     
       6. Method according to  claim 4 , wherein the select control signals applied during the first texture control phase is zero or small, in order to obtain a twisted texture. 
     
     
       7. Method according to  claim 1 , wherein the steps of controlling switching each pixel between the two bistable liquid crystal textures comprise a sequence made up of a first phase and more than two texture control phases, the first phase and all the texture control phases being separated by respective intermediate phases. 
     
     
       8. Method according to  claim 1 , wherein the control signals are of square-wave type to obtain a twisted texture. 
     
     
       9. Method according to  claim 1 , wherein the intermediate phase is designed to control the variation of the falling edge of each control signal, to obtain a uniform texture. 
     
     
       10. Method according to  claim 1 , wherein the transistors have a degraded internal resistance in the off state. 
     
     
       11. Method according to  claim 1 , wherein the control signal is maintained at zero for the pixels whose state does not have to be modified. 
     
     
       12. Method according to  claim 1 , wherein the amplitude of control signals is adapted to obtain grey levels. 
     
     
       13. Method according to  claim 1 , wherein the amplitude of control signals is adapted to obtain, after anchoring breaking, mixed textures in which the two bistable textures coexist in a controlled proportion within one same pixel, these two textures being separated by 180° disclination lines volumewise or by 180° reorientation walls on one of the surfaces, and said method comprising the step of stabilization of the mixed textures by transformation of volume lines into surfaces walls and the immobilization of these walls on the surface.